Coded tone multiplexed alarm transmission system

ABSTRACT

A frequency multiplexed alarm transmission system for transmitting alarm information provided by a plurality of intrusion detectors to a central monitor includes a plurality of alarm detector circuits, each individually associated with one of the intrusion detectors and each selectively operable in an access or a secure mode to provide a line supervision code comprised of a selected sequence of tone signals of a first or a second pair of frequencies, respectively, and a control circuit controlled by the corresponding intrusion detector to modify the tone sequence being provided by the associated alarm detector to indicate an intrusion of a corresponding protected area, different alarm detector circuits providing tone sequences having signals of different frequencies to enable simultaneous transmission of the line supervision codes to the central monitor over a common transmission line, line monitoring circuits at the central monitor include an individual tone detector for each alarm detector circuit for receiving the line supervision code sequences provided by the associated alarm detector circuit, the tone detectors being responsive to a modified line supervision code of the first pair of frequencies to indicate the detection of an authorized intrusion of the corresponding protected area and responsive to a modified line supervision code of the second pair of frequencies to indicate the detection of an unauthorized intrusion of the corresponding protected area.

United States Patent Donovan et al.

[ CODED TONE MULTIPLEXED ALARM TRANSMISSION SYSTEM Inventors: John. C. Donovan, Whitefish Bay;

Carl F. Klein, Milwaukee; Lawrenc'e B. Korta, Milwaukee; Ramesh Krishnaiyer, Milwaukee, all of Wis.

[73] Assignee: Johnson Service Company, Milwaukee, Wis.

[22] Filed: July 17, 1972 121 App], 190.; 272,542

Primary ExaminerThomas B. Habecker Attorney, Agent, or Firm-Johnson, Dienner, Emrich, Verbeck & Wagner [57] ABSTRACT A frequency multiplexed alarm transmission system ALARM DETECT CIRCUIT 3/ 1451 Feb. 12, 1974 for transmitting alarm information provided by a plurality of intrusion detectors to a central monitor includes a plurality of alarm detector circuits, each individually associated with one of the intrusion detectors and each selectively operable in an access or a secure mode to provide a line supervision code comprised of a selected sequence of tone signals of a first or a second pair of frequencies, respectively, and a control circuit controlled by the corresponding intrusion detector to modify the tone sequence being provided by the associated alarm detector to indicate an intrusion of a corresponding protected area, different alarm detector circuits providing tone sequences having signals of different frequencies to enable simultaneous transmission of the line supervision codes to the central monitor over a common transmission line, line monitoring circuits at the central monitor include an individual tone detector for each alarm detector circuit for receiving the line supervision code sequences provided by the associated alarm detector circuit, the tone detectors being responsive to a modified-line supervision code of the first pair of frequencies to indicate the detection of an authorized intrusion of the corresponding protected area and responsive to a modified line supervision code of the second pair of rized intrusion of the corresponding protected area.

26 Claims, 6 Drawing Figures L t. CHANNEL DETECT CIRCUIT 3Q A/so 55 A RM1 0 IV RII I l t, I I I J I 47 4a; I I 1 it I 1 I 142 22 L G I I6 I I I ALARM ozrscr CIRCUIT lg CHANNEL 49 osrscr I I I TONE CIRCUIT L 52 I I 1 egg/ELI c42 I I 1 39 I IEHIL ALARM om'cr CIRCUIT egg/ 54 I 33 CIRCUIT I I3 I I 4a 53 I fifi I I TEST 14/ I40 I I I8 TONE I I I 64 mirror ALARM osrscr CIRCUIT CHANNEL I 34 DETECT I clggulr -I "L l l 30 ALARMsou cE MOVITORZQ I TRANS LINE MM. 22

PMWEDFEBZQ" 3,792,470

SHE-U 3 0F 4 F163 CODE A GEN. (35) ALARM +v 3 LINE GATE 0 CKT (37) TONE Fl GEN Fl F] (36) F0 F0 F0 F0 TONE DET REF F A CODE GEN (46) CODE G GGMHARATGR F165 000E I A GEN (351 TOME F, B GEN '51 F1 Fl F! F 0 F0 F0 F0 F0 F0 F0 F0 F0 F0 SECURE AGGEss Fl Fl Fl Fl Fl Fl TONE CODED TONE MULTIPLEXED ALARM TRANSMISSION SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to remote alarm monitoring systems, and, more particularly, to a frequency multi- In security systems, it is common to transmit alarm signals from a plurality of remote alarm sensors to a central alarm monitor over separate transmission lines. The alarm transmission lines which are connected between protected areas and the central monitor are traditionally the part of the alarm transmission system which is most susceptible to attack or comprise. A variety of approaches have been employed in the past in an attempt to ensure that tampering with these lines'has not occurred.

Alarm line security systems presently available have employed various line protection techniques ranging from the use of DC line monitoring currentscontinuously flowing in the alarm lines to the use of sophisticated cryptographic methods.

Until recently, the use of closed loop DC current systems sensitive to changes in excess of 40 percent of the line monitoring current were the industry standard. Further systems that provide AC pulses superimposed on a standard UL-approved closed DC loop are also presently available. However, both types of systems can be compromised by voltage as well as resistance substitution.

A more recently developed system is based on the principal of a balanced bridge circuit. In its simplest form, this system merely employs a sensitivev indicator connected in a balanced bridge circuit. Although when properly applied the balanced bridge systems provide a very high degree of line security in commercially made line security equipment, such systems are susceptible to frequent false alarms caused, for example, by variations in the line monitoring signal resulting from environmental changes, component aging, noise signals, and the like.

Other techniques which include the use of highly complicated cryptographic techniques, may render the transmission line tamper proof for an extended time. However, the complexity of operation and maintenance, as Well as the initial cost, dictate against the use of such systems.

The problems associated with these prior art systems point to the need for a simple, inexpensive fail-safe system for supervision of alarm transmission lines which extend from alarm sensors in protected areas to a central monitor.

In addition to the need for providing protection for all of the transmission lines connected between the alarm sensors and,the central monitor the cost of the required individual pairs of transmission lines can be significant if the transmission distances are great. Thus, when there are several protected areas in close proximity to one another, but relatively remote from the central monitor, it would seem reasonable to use a system including a multiplexing arrangement to compress the several channels of alarm information into a form which can be carried on a transmission line comprised of a single pair of wires.

SUMMARY OF THE INVENTION The present invention provides a frequency multiplexed alarm transmission system wherein alarm information from a plurality of alarm sensors is transmitted simultaneously over a common alarm transmission line to. a central monitor. In addition, the system provides for the security of the alarm transmission line through the provision of continuous line supervision codes which are generated at the locations of the alarm sensors and transmitted over the transmission line to the central monitor.

In accordance with one embodiment of the invention, the frequency multiplexed alarm transmission system includes alarm source monitoring means for monitoring the status of a plurality of alarm sensors. The alarm sensors may, for example, be intrusion detectors, each of which is located in a different area to be protected and each operable to provide an alarm output responsive to an unauthorized intrusion of the area protected by such alarm sensor.

The alarm source monitoring means includes a plurality of alarm detecting means, including a separate alarm detecting means for each alarm sensor, for monitoring the status of the associated alarm sensor and for transmitting to the central monitor alarm information in the form of a continuous sequence of tone signals coded to represent the condition of an associated alarm sensor. The coded tone sequences provided by all of the alarm detecting means are transmitted to the central monitor over a common transmission line.

The alarm detecting means associated with each alarm sensor includes code generating means, tone generating means and gating means. The code generating means generates a line supervision code comprised of a selected sequence of code bits. Under normal conditions, that is, whenever the corresponding alarm sensor is not providing an alarm output, the line supervision code sequence is passed over the gating means to the tone generating means. The tone generating means is responsive to the bits of the line supervision code to provide output signals of first and second frequencies in a sequence coded to represent the line supervision code.

On the other hand, the gating means is responsive to an alarm output provided by the corresponding alarm sensor to effect modification of the tone sequence provided by the tone generating means.

The tone sequence provided by each alarm detecting means is passed to the transmission line for transmission to transmission line monitoring circuits at the central monitor.

In order to permit simultaneous transmission of the tone sequences provided by all of the alarm detecting means which comprise the alarm source monitoring circuits, the tone generating means of different alarm detecting means provide dual-frequency output signals at different frequencies. Thus, the system provides a plurality of frequency-divided information channels each of which carries alarm information representing the status of a different alarm sensor.

The transmission line monitoring circuits at the central monitor comprise channel separation filter means for separating the tone sequences provided by the alarm detecting circuits into separate channels and plurality of channel detecting means including separate channel detecting means for each alarm detecting means to receive the tone sequences provided by the corresponding alarm detecting means. Each of the channel detecting means includes a reference code generator means which generates a sequence of coded bits which is identical to the line supervision code sequence provided by the code generator means of the corresponding alarm detecting means.

Each of the channel detecting means further includes a tone detecting means for converting the received tones into logic level signals and means which compare the bits of the received code with the bits of the reference code. Under normal conditions (that is when all protected areas are secured), the sequence of bits received by each of the channel detecting means from the corresponding alarm detecting means will be identical with the sequence of bits provided by the reference means of such channel detecting means. However, whenever an alarm output is provided by an alarm sensor, the modified line supervision code sequence provided by the means associated with such alarm detecting alarm sensor will differ from the reference code provided by the associated channel detecting means. Responsive to the detection of a difference in one or more of the bits by a code comparator means, such code comparator means is operable to enable an alarm indication to be registered at the central monitor.

In accordance with a feature of the invention, each of the alarm detecting means is operable in a secure mode and in an access mode. Whenever the alarm detecting means is operable in the secure mode, human movement within the area protected by the corresponding alarm sensor will cause an alarm indication to be registered at the central monitor. On the other hand, whenever the alarm detecting means is operable in the access mode, an indication will be provided at the central monitor that human movement within the area protected by the alarm sensor associated with such alarm detecting means is that of an authorized person.

Each alarm detecting means has associated therewith an access-secure switch means which controls the tone generator means to provide distinct output tone sequences for the access mode and the secure mode. The tone generator means may, for example, comprise trilevel oscillator means controlled by the access-secure switch means to provide a tone sequence of signals of a first pair of frequencies representing the line supervision code whenever the alarm detecting means is operable'in the secure mode and to provide a tone sequence of signals of a second pair of frequencies representing the line supervision code whenever the alarm detecting means is operable in the access mode.

Each of the channel detecting means at the central monitor include access-secure detecting means responsive to a line supervision code comprised of signals of the first pair of frequencies to indicate the corresponding alarm detecting means is operating in the secure mode and responsive to a line supervision code comprised of signals of the second pair of frequencies to indicate that the corresponding alarm detecting means is operating in the access mode.

In accordance with a further feature of the invention, the central monitor includes test generator means for generating a test signal for transmission to the locations of all of the alarm sensors. Each of the alarm detecting means includes motion simulator means responsive to the test signal for providing an output signal representing simulated motion within a given protected area. Such output signal is detected by the alarm sensor which protects such area causing an alarm output to be provided and an alarm indicating code to be generated by the corresponding alarm detecting means, thereby enabling remote testing of all of the alarm sensors from the central monitor.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a frequency multiplexed alarm transmission system provided by the present invention;

FIG. 2 is a block diagram of an alarm detecting circuit and associated alarm sensor of the system shown in FIG. 1;

FIG. 2a is a schematic representation of an alternate gating circuit for the alarm detecting circuit shown in FIG. 2;

FIG. 3 is a timing chart showing the relationship between signals provided by the circuits for the alarm transmission system shown in FIG. 1;

FIG. 4 is a block diagram of a channel detecting circuit of the transmission line monitoring circuit of the alarm transmission system shown in FIG. 1; and

FIG. 5 is a timing chart showing the relationships between signals provided by circuits of the alarm transmission system for the access and the secure modes.

DESCRIPTION OF A PREFERRED EMBODIMENT General Description Referring to FIG. 1, there is shown a block diagram of an exemplary embodiment of a coded tone multiplexed alarm transmission system provided by the present invention. The alarm transmission system includes a plurality of alarm sources 10. In the present example, it is assumed that 20 alarm sources are being monitored. Each of the 20 alarm sources 10 includes an alarm sensor, such as alarm sensors 11-14 shown in FIG. 1, for providing alarm indications for transmission via alarm source monitoring circuits 20 and a transmission line 21 to transmission line monitoring circuits 22 at a central monitor.

Each of the alarm sensors, such as alarm sensors 11, is located in a different area to be protected. Typically the protected areas may be rooms of a building. Each alarm sensor may, for example, be an intrusion detector, operable to provide an alarm output in response to the detection of human motion within the area protected by such alarm sensor. The alarm sources 10 are connected over DC supervised alarm lines, including alarm lines 15-18, to alarm detecting circuits 30 including detecting circuits 31-34, of the alarm source monitoring circuits 20.

Each of the alarm sensors, such as alarrn sensors 11-14 shown in FIG. 1, controls an associated alarm detecting circuit. Thus, for example, alarm sensors ll-l4 are connected over alarm lines 15-18, respectively, to associated alarm detecting circuits 31-34, respectively. The alarm source monitoring circuits 20 including alarm detecting circuits 31-34, may, for example, be located within one of the protected areas such as the area protected by alarm sensor 11.

Each alarm detecting circuit, such as detecting circuit 3l, includes a line supervision code generator 35, a tone generator 36, and a gate circuit 37.

Each line supervision code generator, such as code generator 35, generates a continuous line supervision code comprised of a known sequence of coded logic level bits. Under normal conditions, that is, when no alarm is being provided by the alarm sensor 11, the code bits provided by code generator 35 are passed through the gate circuit 37 to the tone generator 36. The tone generator 36 is responsive to each logic 0 and logic 1 level bit of the line code bit sequence provided by code generator 35 to generate tone signals of first and second frequencies, respectively, thereby providing a sequence of tones coded to represent the bits of the line supervision code. The coded tone sequence is transmitted over the transmission line 21 to the central monitor. The transmission line 21 may, for example, be a standard 4 KHZ bandwidth telephone line.

In the event of the detection of human movement within the protected area, an alarm output provided by sensor 11 over alarm line 15 enables the gate circuit 37 to modify the tone sequence output provided by the tone generator 36 to indicate an alarm. For example, the gate circuit 37 may be responsive to an alarm output to inhibit the passage of bits of the line code provided by code generator 35 to the tone generator 36 whereby the tone generator 36 will generate a continuous tone ofa frequency F1 to indicate that an alarm has been provided by alarm sensor 11. The line supervision code, which is continuously generated, serves to indicate that the transmission line 21, which is connected between the location of the alarm sources 10 and the control monitor, is secure, and also provides a medium for the transmission of alarm information to the central monitor.

The alarm detecting circuits 32-34, which are associated with alarm sensors 12-14, respectively, are similar to alarm detecting circuit 31. Each of the alarm detecting circuits 32-34 includes a code generator, a tone generator, and a gate circuit. However, in order to allow simultaneous transmission over a common transmission line 21 of the alarm information provided by all of the alarm sensors, including alarm sensors 11-14, which comprise the alarm sources 10, different base frequencies are assigned to the tone generators of the alarm detecting circuits 30.

The base frequencies for the tone generators of the alarm detecting circuits 30 are maximally distributed over a predetermined frequency bandwidth which, for example, may be 500 Hz to 3 KHz in order to be compatible with the bandwidth of the transmission line 21. In the exemplary embodiment wherein the conditions of alarm sensors, such as alarm sensors 11-14, are

to be monitored, 20 alarm detecting circuits, such asdetecting circuits 31-34, would be provided. In such case, the base frequencies F1, F2 F20 for the 20 tone generators would be distributed over a frequency band of 2.5 KHz, resulting in a typical channel separation of 120 Hz. Thus, for example, tone generator 36 of alarm detecting circuit 31 may have a base frequency of 550 Hz. The tone generators associated with alarm detecting circuits 32-34 may have base frequencies of 670 Hz, 2,710 112 and 2,830 Hz, respectively.

In addition, to provide a dual frequency output, the base frequency of each tone generator is shifted by a predetermined frequency F0, which may be Hz, under the control of an associated gate circuit. Thus, for example, tone generator 36 provides output signals at the assigned base frequency F1, (550Hz) whenever a logic 0 level output is provided by the associated gate circuit 37 and output signals at a frequency Fl F0, (580 Hz), whenever a logic 1 level output is provided by gate circuit 37.

The coded tone signal output sequences provided by all the alarm detecting circuits 30, such as circuits 31-34, are extended to separate inputs of a summing amplifier 38. The output of the summing amplifier 38 is connected to the transmission line 21. Thus, at any instant in time, the transmission line 21 will carry information provided by the 20 alarm detecting circuits 30 in 20 separate frequency channels.

The frequency multiplexed tone sequences provided by the 20 alarm detecting circuits, including alarm detecting circuits 31-34 shown in FIG. 1 of the alarm source monitoring circuits 20, are transmitted over the transmission line 21 to transmission line monitoring circuits 22 at the central monitor.

The transmission line monitoring circuits 22 include a channel separation filter circuit 39 which provides for separation of the tone sequences provided by the 20 alarm detecting circuigs 30, including alarm detecting circuits 31-34, into separate channels. The alarm line monitoring circuits 22 further include a plurality of channel detector circuits 40, 20 in the present example, including channel detector circuits 41-44 shown in FIG. 1. A separate channel detecting circuit is provided for each of the twenty alarm detecting circuits 30. Thus, for example, channel detecting circuits 41-44 are provided to receive tone sequences provided by alarm detecting circuits 31-34, respectively.

Each channel detecting circuit, such as channel detecting circuit 41, includes a tone detecting circuit 45, a reference code generator 46, and a code comparator circuit 47. The tone detecting circuit 45 converts the tone signals at frequencies of F1 and F1 F0 passed thereto from the channel filter circuit 39 to logic level signals to provide a sequence of coded logic bits representing the line code.

The reference code generator 46 generates a reference code comprised of a sequence of coded logic level bits in which each bit of the sequence is normally identical with corresponding bits of the line supervision code provided by the corresponding alarm detecting circuits 31.

The bits of the line supervision code received from the alarm source monitoring circuits 20 are compared with bits of the reference code by the code comparator circuits, such as code comparator circuit 47, and under normal conditions, the bits of the line supervision and reference codes which are compared in each channel by by the corresponding channel detecting circuits will be the same, indicating both that the transmission line 21 is secure and that all protected areas are secure.

In the event of an intrusion in one of the protected areas, such as the area protected by alarm sensor 11, the alarm detecting circuit 31 associated with alarm sensor 11 provides a continuous tone output for transmission to the central monitor.

Whenever a continuous tone, indicative of an alarm condition, is received by one of the channel detecting circuits 40 at the central monitor, such as channel detecting circuit 41 associated with alarm detecting circuit 31, the bits of line supervision code received will not compare with the bits of the reference code generated by the reference code generator 46 of channel detecting circuit 41. Consequently, when the continuous tone generated by the tone generator 36 of alarm detecting circuit 31 is received at the central monitor, the difference between the received code and the reference code generated at the central monitor will be indicated by the code comparator circuit 47 which will provide an output for enabling an alarm driver circuit 48. The transmission line monitoring circuits 22 include a plurality of alarm registers 49, such alarm registers 51-54 associated with channel detectors 41-44, respectively. Each alarm register, such as alarm register 51, is controlled by an associated alarm driver circuit 48 to provide an audible and a visual indication whenever an alarm is provided by the corresponding alarm sensor 11.

The synchronization of the operation of the code generators, such as code generator 35 of alarm detecting circuit 31, and the reference code generators, such as reference code generator 46 of the channel detecting circuit 41, is controlled by clock pulses provided by a clock signal generator 50 which generates tone signals of a predetermined frequency and at a predetermined rate. The output of the clock signal generator may be connected to the transmission line 21 at the central monitor.

The tone signals generated by the clock signal generator 50 are conducted over the transmission line 21 to the alarm source monitoring circuits and extended over clock pulse detecting circuit 122 to the code generators of each of the alarm detecting circuits 30, such as code generator of detecting circuit 31. The clock signals are also extended to the channel separation filter circuit 39, which provides a separate signal channel for the clock signals and over a clock pulse detecting circuit 124 to the reference code generator circuits for each of the channel detecting circuits 40, such as reference code generator 46 for channel detecting circuit 41. The code generators of the alarm detecting circuits 30 and the reference code generators of the channel detecting circuits are responsive to each clock pulse to generate a bit of the line supervision or reference codes, respectively.

In the foregoing general description of the alarm transmission systems, the alarm detecting circuits were described as being operable in a secure mode, and as such, were responsive to the detection of any human movement within a protected area to provide alarm information for effecting the registration of an alarm at the central monitor. During certain periods of time, however, it may be necessary to permit movement of an authorized person within a given protected area while the alarm system is energized. For example, when the alarm transmission system is used in an application for monitoring the condition of rooms in a school, an authorized person, such as a janitor or a repairman, should be allowed access to the protected area at night.

Accordingly, each alarm detecting circuit, such as alarm detecting circuit 31, has associated therewith an access-secure switch 61. When the access-secure switch 61 is set in the secure mode, any human movement within the area protected by alarm sensor 11 will be indicative of an unauthorized intruder, and the movements of such intruder will cause the registration of an alarm at the central monitor in the manner described in the foregoing.

However, when the access-secure switch 61 is set to the access condition, the alarm detecting circuit 31 associated with access-secure switch 61 will be operative to provide a modified output code sequence which indicates that movements detected within the area protected by alarm sensor 11 are those of an authorized person.

Thus, for example, the access-secure switch 61 associated with alarm detecting circuit 31 is operable to control the tone generator 36 to provide tone signals of a first pair of frequencies F1 and F1 +F0, whenever the access-secure switch 61 is set in the secure position, and to provide tone signals of a second pair of frequencies F1 and Fl-Fll whenever the access-secure switch 61 is set in the access position.

The access-secure switch 61 is a limited access switch, such as a key-operated authorized entry switch, which is located at the site of the alarm sensor 11, as for example, on a wall outside of a room protected by alarm sensor 11. The access-secure switch 61 has associated therewith a tamper switch having normally closed contacts 66 connected in series with the conductor 67 which connects the output of the tone generator 36 of alarm detector 31 to an input of the summing amplifier 38. Thus, tampering with the access-secure switch 61 will result in the interruption of the transmission of the tone sequence provided by alarm detecting circuit 31, and thus the registration of an alarm at the central monitor.

Each of the channel detecting circuits 40 of the transmission line monitoring circuits 22, such as channel detecting circuit 41, includes an access-secure condition detecting circuit, such as circuit 55 for channel circuit 41. The access-secure detecting circuit 55 is enabled to control the alarm register circuit 51 associated with channel detector 41 to indicate the status of the corresponding access-secure switch 61.

Thus, the access-secure circuit 55 is responsive to a received line monitoring code comprised of tone signals of frequencies F1 and F1 F0 to enable the alarm registration circuit 51 to provide a first indication representing that the corresponding access-secure switch 51 is set in the secure mode. Alternatively, the accesssecure circuit 55 is responsive to a received line supervision code that is comprised of the signals of frequencies F1 and Fl-FO to enable the alarm registration circuit 51 to provide a second indication representing that the corresponding access-secure switch is set in the access mode.

Although movements of an authorized person within a given protected area will be detected by the alarm sensor in such area, and an alarm output code will be provided by the alarm monitoring circuit associated with such alarm sensor, the access-secure indication provided at the central monitor will indicate that such movements are those of authorized person.

Detailed Description Referring to FIG. 2, there is shown a block diagram of the alarm detecting circuit 31 which is associated with the alarm sensor 1 1. The alarm sensor 11, may, for example, comprise a microwave motion detector 70 which may be of the type which is conventional in the art. The motion detector 70 includes a transmitter 71, a receiver 72, and an alarm signal detecting circuit 73. The transmitter 71 generates microwave signals at a predetermined frequency for radiation via a transmitter antenna 74 into the protected area 75. Such radiated signals will be reflected off objects within the protected area 75 and the reflected signals will be received by an antenna 78 of the receiver 72. When the transmitted signals are reflected off a human intruder, indicated at 76, moving within the protected area, the frequency of the received signals will be shifted from the frequency of the transmitted signals providing Doppler signals which are related in characteristics to an object such as a human intruder 76, moving within the protected area 75. When the moving object is a human intruder, the Doppler signals detected by the receiver 72 will be passed to the alarm signal detecting circuit 73 which will correspondingly provide an alarm output over alarm line 15.

The alarm sensor 11 is connected over the DC supervised line 15 to the alarm line condition monitoring circuits 31. The alarm line 15 normally conducts a signal of predetermined voltage +V, representing a logic 1 level. In the event of an alarm output provided by the alarm sensor 11, the voltage on alarm line 15 will go to ground representing a logic level. It is pointed out that the alarm line 15 is DC supervised so as to provide for an alarm in the event of a change of more than 5 percent in the DC current which continuously flows in the alarm line 15. Thus, an open or short circuit of the alarm line will immediately result in the transmission of an alarm indication to the central monitor.

The output of the alarm sensor 11 is extended over the alarm line 15 to an input of the gate circuit 37 which, in accordance with one embodiment, may comprise an AND gate 81. A second input to AND gate 81 is provided by the output of the code generator 35.

The code generator 35 generates a known sequence of random bits which form a line monitoring code for the transmission line 21 which carries alarm information from the alarm source monitoring circuits to the transmission line monitoring circuits 22.

One code generator suitable for this purpose is described in the copending application US. Pat. Ser. No. 193,450 of John C. Donovan, Ramesh Krishnaiyer and Frank J. Esser, which was filed on Oct. 28, 1971.

In an exemplary embodiment, the code generator 36 includes a four stage shift register 85 having feedback connections over conductors 86 and 87 from the first and fourth stages, respectively, connected through an Exclusive OR circuit 88 to the input of the first stage. The Exclusive OR circuit 88 provides a logic 0 output whenever the two inputs to the Exclusive OR circuit arethe same logic level, and provides a logic 1 output whenever the inputs are different logic levels. Code generator 35 preferably includes at least a 16 stage register and may be as much as a 32 stage register capable of generating a pseudo-random sequence of bits, the length of the sequence being given by the relationship 2-1, where N is the number of stages which comprise the shift register of the code generator 35. In the present example, for convenience only, a four stage register 85 is shown, and 15 bits are provided.

The register 85 is closed at a predetermined rate by clock signals supplied over transmission line 21 by clock signal generator 50 to provide a pseudo-random sequence of bits which control the tone generator 36 to provide tone signals of first and second frequencies in a sequence coded to represent the bits of the pseudo random code.

'Digressing, in the present illustration, the lowest frequency to be transmitted over the line 21 from the alarm source monitoring circuits 20 to the central monitor is 520 Hz. To allow at least four cycles of this frequency signal to be transmitted, the clock frequency can be no higher than 520 I'Iz/4 or approximately 130 Hz. However, the frequency of 130 Hz is below the bandwidth of the transmission line 21 used in the present example, and thus, this clock frequency 130 Hz cannot be transmitted directly.

Accordingly, the frequency of the clock signals generated by the clock signal generator 50 may, for example, be 2,950 Hz to be within the system passband. The clock signals at 2,950 Hz are transmitted over line 21 to both the protected areas and the monitoring areas, and the frequency of the transmitted clock signals is divided down in both the protected areas and the monitoring area to provide the desired clock frequency.

Thus, referring to FIG. 2, the clock signals at 2,950 Hz provided by clock signal generator 50 and transmitted over line 21 to the protected areas are received by the clock signal detecting circuit 122. The clock signal detecting circuit 122 includes a clock signal filter 90, a clock signal detector 91, and a clock signal divider circuit 92.

The tone filter 90 has a pass band of approximately 2,900 Hz to 3,000 Hz so as to pass only signals at a frequency of 2,950 Hz provided by the clock signal generating circuit 50. The output of the bandpass filter circuit 90 is connected to the input of the detector circuit 91 which converts the signals into logic level pulses which in turn are extended to the clock signal divider circuit 92. The clock signal divider circuit 92 divides the transmitted clock frequency at 2,950 I-Iz by a factor of 24, for example, to provide an effective clock frequency of approximately 123 Hz.

Similarly, referring to FIG. 4, the clock pulse detecting circuit 124 of the line monitoring circuits 40 includes a clock signal detecting circuit 108 and a clock signal divider circuit 109. The clock signal divider circuit 109 divides the transmitted clock signal at 2,950 Hz by a factor of 24 to provide clock pulses at a rate of approximately I23 Hz for the circuits of the central monitoring circuits 40.

The code generators of the alarm detecting circuits 30, and the reference code generators of the channel detecting circuits 40 are responsive to each clock pulse at the I23 Hz rate to generate a bit of the line supervisory and reference codes, respectively.

Referring again to FIG. 2, to illustrate the operation of the code generator 35, it is assumed that initially all stages of the register store logic 1 level bits and that the register 85 is thereafter cycled under the control of clock pulses supplied over transmission line 21 and clock signal detecting circuit 122. The sequence of words given in Table I will appear in the stages of the shift register 85 as successive clock pulses are provided.

TABLE I Register Stage I 2 3 4 (Initial) I I I I Clock Pulse 1 O l l l 2 I O I I 3 0 I 0 I 4 I 0 I 0 5 I I 0 I 6 0 I I 0 7 0 O l l 8 l 0 0 I 9 0 l 0 0 I0 0 0 I 0 I I 0 0 O I I2 I 0 0 0 I3 I I 0 0 I4 I I I 0 Since initially, the first and fourth stages of register 85 both contain binary ls, the Exclusive OR circuit 88 provides a logic output which is gated into the first stage of the shift register 85 with receipt of the first clock pulse as can be seen in step 1. The clock pulse also shifts the logic ls from the first through third to the second through fourth stages, respectively. When the second clock pulse is received, the outputs of first and fourth stages are different, and accordingly the Exclusive OR circuit 88 will provide a logic 1 output which will be gated into first stage of the shift register 85 as the bits in the first through third stages are gated into the second through fourth stages, respectively. In a similar fashion for the third through fourteenth pulses, outputs fed back to the input of the first stage cause the sequence of words given in Table I to be generated. The sequence given in Table I repeats after the 15th clock pulse.

Thus, it is seen that 15 pseudo-random words are provided when the number of stages of the shift register is equal to four. The total number of combinations of N bitsisZ so only one combination is missing.The

rnissing m ultibit wordds OOIjO whic lTcan be seen to repeat itself or have a trivial cycle of one in the code generator circuit 35. The random bit sequence generated by the code generator circuit 35 is extended to the input 83 of the AND gate 81. A portion of the sequence thus provided is shown in line A of FIG. 3.

Under normal conditions, that is, when no alarm output is being provided by alarm sensor 11, the +V (logic 1) level (FIG. 3, line B) on the alarm line 15 is extended to input 82 of AND gate 81. Accordingly, whenever input 82 is at +V level, AND gate 81 will be enabled to provide a logic 1 level output response to each logic 1 level bit of the line monitoring code and will be disabled to provide a logic 0 level output responsive to each logic 0 level bit of the line supervision code. The logic 1 and logic 0 level outputs of AND gate 81, shown in line C of FIG. 3, control the tone generator circuit 36.

The tone generator circuit 36 includes a tri-level oscillator circuit 95 having an associated bias supply circuit 96. The oscillator circuit 95 is operable to provide a first or a second dual frequency output as a function of bias conditions for the oscillator circuit 95, as determined by the bias supplied to the oscillator circuit 95 by the bias supply circuit 96, and the level (+V or 0) of the input signal supplied to the input of the tone generator circuit 36 by the code generator 35.

The tri-level oscillator circuit 95 may comprise a standard FSK (frequency shift keying) tone transmitter. One commercially available tri-level FSK tone transmitter is the Model PIE-17845 FS, manufactured by RFL Industries of Boonton, NJ.

The FSK oscillator circuit 95 has two input terminals 95a and 95b. Whenever the signal levels at both inputs 95a and 95b are logic 0 or ground levels, the oscillator 95 will provide an output signal at the base frequency F 1, 550Hz in the present example, representing a logic 0 level. If an input voltage of volts is applied to one of the inputs, such as input 95a, the oscillator circuit 95 will provide an output signal at a frequency Fl F0, 580 Hz. On the other hand, if a 10 volt input signal is applied to input 95b an output signal at a frequency F1 F0, 520 Hz will be provided.

The bias supply circuit 96 serves as an interface betwen the output of the gate circuit 37 and the input of the oscillator circuit to convert the logic 0 and logic 1 levels (0 and +V volts, respectively) to voltage levels (0, V) suitable for controlling the oscillator circuit 95. The bias supply circuit 96 is controlled by the access-secure switch 61 to extend the control outputs of the gate circuit 37 to one input 95a of the oscillator circuit 95 whenever the access-secure switch 61 is set in the secure position, and to extend the control outputs of gate circuit 37 to the other input 95b of the oscillator circuit 95 whenever the access-secure switch 61 is set in the access position. The logic level signal outputs of the gate circuit 37 are converted to voltage levels compatible with the oscillator circuit 95 by the bias supply circuit 96 in accordance with the relationships given in Table II.

TABLE II Output-Gate 37 Osc. Input 95a Osc. Input 9512 Secure Thus, when the access-secure switch 61 is set in the secure position, and the logic zero level is provided by AND gate 81, the bias supply circuit 96 will extend the 0 volt signal to input 95a of the oscillator circuit, which will then provide a signal output of the base frequency F1, (550 Hz in the present example), representing a logic zero level. Additionally, if a logic 1 level input is provided by gate circuit 81, the bias supply circuit 96 will provide a lOV level for input 95a, and the oscillator 95 will produce a signal output of a frequency F1 F0, (580 Hz).

When the access-secure switch 61 is set in the access mode, the oscillator circuit 95 will receive bias at a second level from the bias supply circuit 96. Accordingly, in response to each logic 0 level input, the bias supply circuit 96 will provide the logic I 0V level input to the oscillator input 95b and the oscillator will provide a signal output of a frequency F l-FO (520 Hz). Moreover, in response to a logic 1 level output provided by gate circuit 81, the bias supply circuit 96 will provide an output at 0V volts to input 95b of the oscillator circuit 95 such that the oscillator circuit will provide a signal at the base frequency Fl (5501-12). The above conditions are summarized in Table III.

TABLE III Code Generator Output Frequency Output Access Secure Logic 0 Fl-FO Fl Logic l F1 F] F0 Thus, in the present example, for the random code sequence shown in line A of FIG. 3, the tone generator 36 will provide the tone output sequence shown in line D of FIG. 3 when the access-secure switch 61 is set in the secure position.

In response to an alarm output on line 15, gate circuit 37 will be disabled for the duration of such alarm output, and the tone generator 36 will provide a continuous tone output at the base frequency F1.

Digressing, an alternative gate circuit 37' is shown in FIG. 2A. The gate 37' comprises an Exclusive OR circuit 81. The alarm line 15 is connected over an inverter 81a to one input 82' of the Exclusive OR circuit 8.1 A second input 83 of the Exclusive OR circuit 81 is connected to the output of the code generator 35. The output of the Exclusive OR circuit 81' is connected to the input of the tone generator circuit 36.

The Exclusive OR gate circuit 81 is operative to normally pass the output of the code generator 35 unaltered to the tone generator 36 and, in the event of an alarm provided by alarm sensor 11, to invert each bit of the line supervision code as the sequence of coded bits is passed to the tone generator 36.

The Exclusive OR circuit 81' provides a logic level output whenever the signal levels at both inputs 82' and 83 are at the same level and a logic 1 level output whenever the signal levels at both inputs 82 and 83' are at different levels.

Thus, under normal conditions, a logic 0 level is provided at input 82 accordingly, the Exclusive OR circuit 81 will provide a logic 0 output for each logic 0 bit of the line supervision code, and a logic 1 output for each logic 1 bit of the line supervision code as shown in Table IV.

TABLE IV (Inverted) Alarm Output 0 0 I I Output-Code Gen. 35 0 l 0 l Output-Gate 37 0 l I 0 In the event of an alarm, a logic 1 level will be provided at input 82' of the Exclusive OR circuit 81, and accordingly, the Exclusive OR circuit 81 will provide a logic 1 output for each logic 0 bit of the line supervision code and a logic 0 output for each logic 1 bit of the line supervision code as shown in Table IV.

Thus, in the event of an alarm condition, the alternative gate circuit 37' will invert each bit of the line supervision code to provide a sequence of coded bits for controlling the tone generator circuit 36. The tone generator circuit 36 will thus be controlled to provide a signal ofa frequency of F1 for each logic 1 level bit of the line supervision code and a signal of a frequency Fl F0 for each logic 0 level bit of the line supervision code.

The output of the tri-Ievel oscillator 95 is extended over a conductor 67 and normally closed contacts 66 of a tamper switch 45, associated with the accesssecure switch 61, to an input 38a of the summing amplifier38. The outputs of other alarm detecting circuits, including circuit 32-34 shown in FIG. 1, are connected to further inputs of the summing amplifier, such as inputs 38b-38d, respectively. The tone sequences representing the line supervision codes provided by each of the detecting circuits, including detecting circuit 31 are passed over the summing amplifier 38 to the transmission line 21 for transmission to the central monitor.

Transmission Line Monitoring Circuits Referring to FIG. 4, the transmission line monitoring circuits 22 include a channel separation filter circuit 39 which receives the frequency multiplexed tone sequences provided by the alarm detector circuits 30.

The output of the tone filter 101 is connected to an associated channel detecting circuit 41. Each channel detecting circuit, such as circuit 41, includes a tone detecting circuit 45.

The tone detecting circuit 45 includes circuitry for amplifying and detecting the received signals, providing logic level outputs coded to represent the tones of the coded tone sequence received.

One multi-frequency receiver which is compatible with the tone transmitter 37 used in the alarm detecting circuits 30 is the model I-IB-208l0 F S receiver, and the associated model FIB-20830 base frequency detector, which is manufactured by RFL Industries of Boonton, NJ. It is pointed out that this commercially available receiver includes an input filter stage, and accordingly, the channel separation filters, such as filter 101 associated with tone detecting circuit 45, for example, would not be required.

The tone detecting circuit 45 may include an amplifier stage for amplifying the received signal, and a discriminator stage which is operable to convert the tone signals to logic levels. The discriminator circuit may consist of a first series resonant circuit tuned near the frequency F l F0, a second series resonant circuit tuned near the frequency F l-F0 and a circuit for detecting the base frequency F1.

Whenever a signal at a frequency F1 F0 is received, the tone detecting circuit 45 will provide a signal at +V level at an output 45a. In addition, the tone detecting circuit 45 will be responsive to signals at frequencies F1 or Fl-F0 to provide signals at 0V and V levels, respectively over respective outputs 45b and 450.

Thus, when the corresponding alarm detecting cir cuit 31 is operating in the secure mode, providing a coded tone sequence of signals at frequencies F l and F1 F0, the tone detector 45 will provide signals at 0V or +V levels in correspondence with the frequency of the received signal at outputs 45a and 45b, respectively, as shown in the portion of line C of FIG. 5 for the secure condition. On the other hand, when alarm detecting circuit 31 is operating in the access mode, providing a coded tone sequence of signals at frequencies F l F0 and F 1, the tone detector 45 will provide signals at V or 0V levels at outputs 450 of 45b, respectively, as shown in the portion of line C of FIG. 5 for the access condition.

The tone detector circuit 45 may include an interface circuit 45d having inputs connected to outputs 450-450 of the tone detector circuit to convert the three voltage levels +V, 0, V into a two level code(+V and 0V)at outputs 45e and 45f of the tone detector circuit 45 representing the logic 1 and logic 0 bits of the transmitted code sequence. The interface circuit 45d may, for example, comprise a plurality of relays such that the output signals can be contact condition changes.

The outputs 45e and 45f of the tone detector circuit 45 are extended over an OR gate 116 to an input 47a of the code comparator circuit 47. The code comparator circuit 47 compares the bits of the received sequence of code bits with corresponding bits of a reference code provided by a reference code generator 46 supplied over a second input 47b of the code comparator circuit 47.

The reference code generator 46 is similar to the code generators, such as code generator 35, of the alarm source monitoring circuits 20, and operates in sequence with such line code to provide a sequence of random bits (shown in FIG. 3, line F) in which each bit is normally identical with a corresponding bit of the random bit sequence (shown in FIG. 3, line A) at any given time. The reference code generator 46 comprises a four-stage shift register having outputs of stages l and 4 fed back over an Exclusive OR circuit 111 to the input of the first stage.

The reference code generator 46 is driven by clock pulses at the 123 Hz rate provided over clock pulse detector circuit 124 from the clock signal generator 50, to provide the sequence of random words shown in Table I that is provided by the code generator 35.

The clock signals at a frequency of 2,950 Hz provided by the signal generator 50 are conducted over the transmission line 21 to the input of the channel separation filter circuit 39, which includes a clock signal bandpass filter circuit 107 having a pass band of approximately 2,900 Hz to 3,000 Hz. The output of the bandpass filter circuit 107 is connected to an input of a clock signal detecting circuit 124 which includes a detecting circuit 108 for converting the signals provided by the clock signal generator 50 to logic level pulses and a pulse divider circuit 109 which divides the clock rate of the received clock pulses by 24, providing clock pulses at a 123 Hz rate. Such clock pulses at the 123 Hz rate are extended to the reference code generators, such as code generator 46, to effect the generation of a reference code which is supplied to the code comparator circuit 47.

The code comparator circuit 47 includes a pair of data register and conditioning circuits 112 and 113, an Exclusive OR gate circuit 114, and an output gating circuit 115. The data register circuits 1 l2 and 113 may comprise flip-flops, timing and delay circuits, and accessory gates which receive the code bits from the reference code generator 46 and from the alarm detecting circuit 31 at the corresponding remote protected area. The data register 112 has an input 112a connected to the output of OR gate 116 to receive the bits of the code transmitted to the central monitor from alarm detecting circuit 31. The data register 113 has an input 113a connected to the output of the reference code generator 46. in addition, inputs 112b and 1l3b of data register circuits 112 and 113, respectively are connected to the output of the clock signal detecting circuit 124 provided at the output of signal detecting circuit 124.

The data register circuit 112 is operable to store the bits received from the protected area associated with alarm detecting circuit 31, and the data register circuit 113 is operable to store the bits from the reference code generator 46 until the bits provided from both sources have been received. Thereafter, the data register circuits 112 and 113 are enabled hy clock pulses, appropriately delayed by delay circuits of the data register circuits 112 and 113 to shift the bits to individual inputs of the Exclusive OR circuit 114 where the actual bit comparison takes place. The output of the Exclusive OR circuit 114 is extended to an input of an AND gate 115a of the output gating circuit 115. A second input of AND gate 115 receives enabling clock pulses provided over a time delay circuit 115b and a monostable circuit 1150 from the clock pulse detecting circuit 124.

The purpose of AND gate 115a is to allow an input to the alarm driver circuit 48 only during the interval of comparison. AND gate 115a is enabled by a clock pulse during the middle of the interval of comparison to minimize the affects of transients which may occur at the beginning or end of the comparison period. The enabling clock pulses provided at the output of clock pulse detecting circuit 124 are extended to the time delay circuit 1l5b which enables a monostable circuit 1150 for providing an enabling input for AND gate a. The output of the monostable circuit 1l5c is a single pulse, shorter in duration than the clock pulse, which is provided during the middle of the interval of comparison.

The AND gate 115a of the code comparator circuit 47 is enabled to provide a logic 1 level output, as shown on line G of FIG. 3, whenever corresponding bits of the reference code and the line supervisory code received from the alarm detecting circuits 31 are different. The AND gate 1 15a, when enabled, sets a latch circuit 1 15d which, in turn, enables an associated alarm register 51 to provide an audible and a visual alarm indication at the central monitor. The latch circuit 1 15d may, for example, be a flip flop which is set to sustain an alarm signal until acknowledged. An acknowledge switch ll5e enables manual reset of the latch circuit 115d.

Under normal conditions, as each bit of the line code received from alarm detecting circuit 31 is extended over bandpass filter 101 of the channel separation filter circuit 39 and tone detector circuit 45 to the input 112a of the data register 112, the corresponding bit of reference code register generator 46 and extended to the input 113a of data register 113, will be identical with the bit extended to data register 112. The bits provided at the inputs 112a, 113a of data register circuits 112 and 113 are clocked to the Exclusive OR circuit 114 by the clock pulse which effects the generation of the line supervision code and reference code bits.

Whenever the bits provided at the outputs of data register circuits 112 and 113 are the same, the Exclusive OR circuit 114 connected will provide a logic 0 level output.

Whenever the alarm detecting circuit 31 is providing an alarm code, that is, either a continuous tone output at the base frequency F1 or an inverted code, the first bit of the alarm code received at the central monitor by channel detecting circuit 41 at the input 112a of data register 112 will be as a different level from the corresponding bit of the reference code generated by reference code generator 46 and supplied to data register 113.

Accordingly, since the bits extended over data register circuits 112 and 113 to the Exclusive OR gate 114 will be different, the Exclusive OR gate 114 will provide a logic 1 level output. The logic 1 level output provided by the Exclusive OR gate 114 is extended to AND gate 115a, which will be enabled when a logic 1 level signal is provided at the second input of AND gate 115a by monostable circuit 1150 When AND gate 115a enabled in response to the receipt of the alarm bit by the channel detecting circuits 41, a logic 1 level output provided by AND gate 115a sets latch circuit 115d which enables the alarm driver 48 to energize an alarm tone generator 1 17, causing an audible alarm tone to be generated and an alarm lamp 118 to be lit.

Access-Secure Detecting Circuit The logic level outputs provided at outputs 45e and 45f of the tone detector 45 are also extended to the access-secure condition detecting circuit 55. The accesssecure detecting circuit 55 includes a flip flop 121 having a set input connected to output 45e of the tone detecting circuit 45 and a reset input connected to output 45f of the tone detecting circuit 45. The positive output Q of flip flop 121 is connected to a secure indication 17 lamp 123 and the negative output 6 of flip flop 121 is connected to an access indicating lamp 125.

In operation, flip flop 121 is set causing lamp 123 to be lit whenever a logic 1 level output (indicating detection of a signal at a frequency F1+F0) is provided at output 45:! of the tone detector circuit 45. The flip flop 121 is reset causing lamp 125 to be lit whenever a logic 1 level output (indicating detection of a signal at a frequency F1F0) is provided at output 45f of the tone detecting circuit 45.

Referring to FIG. 5, it is assumed initially that the ac cess-secure switch 61 (FIG. 2) associated with alarm detecting circuit 31 is set in the secure position and that the line supervision code bit sequence provided by the code generator 35 of alarm detecting circuit 31 is as shown in line A of FIG. 5. Accordingly, the tone generator 36 will provide the sequence of tones of frequencies F1 (logic and F1 +F0 (logic 1) given in the portion of line B of FIG. 5 indicated by the word secure.

The tone detector 45 of channel detecting circuit 41 at the central monitor will be responsive to the tone sequence (FIG. 5, line B) provided by tone generator 36 to provide the sequence of logic level outputs shown on line C of FIG. 5. while the alarm detecting circuit 31 is operating in the secure mode, the tone detector 45 will provide a signal at 0V level at output 45b for each signal at frequency F1, and a signal at +V level at output 45a for each signal at frequency Fl F0. Moreover, the interface circuit 45d will provide an output at a +V level at output 45e whenever an output at +V level is provided at output 45a. Accordingly, flip flop 121 of the access-secure detecting circuit will be set, providing a +V level signal (line D FIG. 5) at the set output thereof, and the secure condition indicating lamp 123 will be lit whenever a signal at a frequency Fl F0 is detected.

When the access-secure switch 61 is set to the access position, the output of the tone generator 36 will be modified such that the tone sequence representing the line supervisory code indicates that the alarm detecting circuit is operating in the access mode. Accordingly, in such case, the tone generator 36 will provide signals at a frequency F1-F0 for each logic 0 bit of the line supervisory code (shown in the latter portion of line A, FIG. 5) and signals at a frequency of F0 for each logic 1 level bit of the line supervisory code. Thus, in the present example, the tone sequence produced when the access-secure switch 61 is set in the access position is shown in the latter position (indicated access) of line B of FIG. 5.

The tone detecting circuit 45 is responsive to the modified tone sequence representing the line supervisory code provided the alarm detecting circuit 31, whenever the detecting circuit 31 is operating in the access mode, to provide an output at V level whenever a signal at frequency F 1-F0 is detected, and an output at 0V level whenever a signal at frequency F0 is detected, as shown in the latter portion of line C, FIG. 5. The tone detecting circuit 45 will provide a logic 1 level output at output 45f in response to the detection of each signal at frequency F1F0.

Accordingly, the flip flop 121 will be reset by the pulse provided at output 45f of the tone detector 45, causing secure condition indicating lamp 123 to be extinguished and access condition indicating lamp 125 to be lit to indicate that the alarm detecting circuit 31 is operating in the access mode, and that movements within the area protected by alarm sensor 1 1 associated with detecting circuit 31 are those of an authorized person.

When the access-secure switch 61 is again set in the secure position, the tone generator 36 will provide tones at frequencies F1 and F1 +F0, and the output provided by the tone detecting circuit 45 responsive to the first tone at frequency F1 F0 will effect the set ting of the flip flop 121, the lighting of secure condition lamp 123, and the extinguishing of lamp 125.

It is pointed out that the status of the access-secure condition indicating lamps 123 and 125, respectively, may not change instantaneously upon the changing of the status of the access-secure switch 61 at a remote location. Depending upon the place in the pseudorandom bit sequence at which the switch status is changed, it may be several cycles before a bit occurs which will cause the generation of a signal at frequency F1 F0 or F1 F0 as is appropriate. However, the maximum delay should be a matter of only a few seconds at most.

It is further noted that while only one secure lamp 123 and one access lamp 125 are shown, security practice frequently calls for redundant display and indicating devices such as lamps 123 and 125 where such devices are subject to failure such as normal burnout.

Moreover, to obtain a higher degree of security, the functional access-secure indicating circuit including the access-secure switch 61, the secure lamp 123, the access lamp 125 and the associated lamp drive circuit 121 may be duplicated at the central monitor, providing a supervisory access-secure indicating circuit for the area protected by alarm sensor 11. The supervisory access-secure indicating circuit would allow a person manning the control panel including the alarm register 49 to duplicate the operation of the functional accesssecure switch 61 at approximately the same time that an authorized person operates such access-secure switch 61. A comparator circuit operable to compare the access-secure conditon indicated by the supervisory access-secure circuit and the condition indicated by functional access-secure circuit would effect generation of an alarm indication whenever the conditions of the two access-secure indicating circuits are different. Additional Features Referring to FIG. 1, an addition to the receiver aspects of the central monitor, the transmission line monitoring circuits 22 at the central monitor includes a remote test signal generator which permits testing of the operation of the alarm sensors such as alarm sensors 11-44, from the central monitor. The remote test signal generator 140 generates a test signal of a predetermined frequency, which is transmitted over the transmission line 21 to the alarm source monitoring circuits 20 and extended to all of the alarm detecting circuits 30.

The alarm source monitoring circuits 20 include a test signal detecting circuit 141 which receives the test signals transmitted over transmission line 21 and provides a control output signal which is extended to each of the alarm detecting circuits 30, such as alarm detecting circuit 31, shown in FIG. 2, to energize a corresponding motion simulation output generating circuit, such as motion simulation output generating circuit 142 of the alarm detecting circuit 31.

The motion simulation output generating circuit 142 is responsive to each control output signal provided by the test signal detecting circuit 141 to produce a signal output representative of human motion for enabling the corresponding alarm sensor 11 to produce alarm output if the alarm sensor 11 is functioning properly. The motion simulation signal may well be a frequency signal, but that frequency is selected for its motion simultation characteristics, and does not necessarily bear any relationship to the frequency of the remote test signal.

Referring to FIG. 2, the test signals transmitted over the transmission line 21 to alarm source monitoring circuit 20 are received by the test signal detecting circuit 141. The test signal detecting circuit 141 includes a bandpass filter I47 and a tone detector circuit 148. The bandpass filter 147 is tuned to pass only signals which are of the frequency of the test signals provided by the test signal generator 140 to the tone detector circuit 148.

The tone detector circuit 148 is responsive to signals passed by the tone filter 147 to provide control signals over conductor 143 for the motion simulation output generating circuit 142. The motion simulation output generating circuit includes a drive circuit 150 and a functional device 149.

In the exemplary system wherein the alarm sensor 11 associated with alarm detecting circuit 31 comprises a microwave motion detector circuit 70, the performance of alarm sensor 11 can be tested through the use of various electrical or mechanical functional devices activated by enabling of the drive circuit 150 by the test signal detector circuit 141. Examples of mechanical motion simulators include a small fan placed in the detection field of the sensor 11 and a piezoelectric element placed within the receiver antenna structure 78. Electrical motion simulators include the application of the low frequency modulation signal to the transmitter oscillator (conveniently, 60 Hz is most appropriate for motion detectors operating in the GHz region as this frequency corresponds to that generated by normal human motion) or the excitation of a small neon bulb within the antenna structure (74, 78). This latter approach is quite convenient in the case of an avalanche diode oscillator which requires dc voltage as high as that used for neon bulb excitation. The movement of the ionized gases in the neon bulb can be then utilized to electrically simulate intruder motion. A diode (point contact, Schottky or PIN structure) can also be placed within the antenna structure and driven at a low (60 Hz or lower is convenient) frequency to modulate the microwave beam as in human motion. Finally, switching transients in the supply circuit of the transmitter 71 can be used to simulate transient motion conditions.

The functional device 149, when activated by the drive circuit 150, thus effects modulation of the microwave signals radiated into the protected area 75, as would occur when such microwave signals are reflected off a human target moving within the protected area. The modulated microwave signals thus produced are detected by the receiver 72 and passed to the alarm signal detecting circuit 73, enabling the alarm signal detecting circuit 73 to provide an alarm output on alarm line 15.

The alarm output thus provided will enable the corresponding alarm detecting circuit 31 to provide an alarm code (that is, a continuous tone output or a complemented line monitoring code) in the manner described hereinabove for transmission to the central monitor.

The receipt at the central monitor of such alarm code in response to the transmission of a test signal from the central monitor will indicate that alarm sensor 11 is functioning properly. On the other hand, a malfunction in the alarm sensor 11 will be indicated by the failure to receive an alarm code after the test signal has been transmitted to the locations of the alarm detecting circuits.

We claim:

1. In an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, alarm detecting means for each of said protected areas, each of said alarm detecting means including output means for continuously generating a line supervisory code comprised of a selected sequence of tone signals of predetermined frequencies, different output means being operable to provide supervisory code sequences having signals of different frequencies to enable simultaneous transmission over said transmission line of the supervisory code tone sequences provided by all of the output means, and separate control means for each output means for controlling an associated output means to modify the supervisory code tone sequence to indicate an alarm for a corresponding protected area, and line monitoring means at said central monitor, including an individual tone detecting means for each of said alarm detecting means including receiver means for receiving the supervisory code tone sequences provided by an associated alarm detecting means and alarm output means responsive to a modified supervisory code tone sequence provided by the associated alarm detecting means to provide an alarm indication at said central monitor.

2. An alarm transmission system as set forth in claim 1 wherein each said output means includes signal generating means for generating tone signals of first and second frequencies in said selected sequence, said signal generating means being controlled by said control means to provide a continuous tone output of one of said frequencies to indicate an alarm.

3. An alarm transmission system as set forth in claim 1 wherein said output means includes code generating means for generating a supervisory code comprised of a selected sequence of coded logic level bits, and tone generating means responsive to the bits of the supervisory code sequence to normally provide tone signals of predetermined frequencies in a sequence coded to represent the bits of the monitor code, and wherein each of said receiver means at said central monitor includes tone converter means for converting the received tone sequence into a sequence of logic level bits, reference code generating means for generating a reference code comprised of a sequence of logic level bits in which each bit is normally identical with a corresponding bit of the supervisory code at any given time, and each of said alarm output means includes comparator means for comparing each bit of the supervisory code with the coresponding bit of the reference code and means for providing an indication whenever the corresponding bits are different.

4. An alarm transmission system as set forth in claim 3 wherein each of said control means includes gating means normally enabled to pass the bits of the supervisory code unaltered to the corresponding output means to effect the generation of the supervisory code tone sequence, said control means further including alarm sensor means operable to provide an alarm output signal indicating an alarm condition in the corresponding protected area, said gating means being responsive to an alarm output signal provided by an associated alarm sensor means to invert each bit of the supervisory code as the bits are passed to the tone generating means to thereby effect the generation of said modified supervisory code tone sequence.

5. In an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, alarm detecting means for each of said protected areas, each of said alarm detecting means including code generating means for continuously generating a line supervisory code comprised of a selected sequence of logic level coded bits, tone generating means responsive to the bits of the supervisory code sequence to normally provide tone signals of at least first and second frequencies in a sequence coded to represent the bits of the supervisory code, and separate control means for each alarm generating means for controlling an associated tone generating means to modify the tone sequence to indicate an alarm for the corresponding protected area, different tone generating means being operable to provide tone signals of different frequencies to enable simultaneous transmission over said transmission line of the tone sequences provided by all of the tone generating means, and line monitoring means at said central monitor, including an individual tone detecting means for each of said alarm detecting means for receiving the tone sequences provided by an associated tone generating means, each of said tone detecting means including tone converter means for converting the received tone sequence into a sequence of logic level bits, reference code generating means for generating a reference code comprised of a sequence of logic level bits in which each bit is normally identical with a corresponding bit of the supervisory code at any given time, and comparator means for comparing each bit of the supervisory code with the corresponding bit of the reference code and providing an indication whenever any of the corresponding bits of the two sequences are different.

6. An alarm transmission system as set forth in claim wherein said line monitoring means further includes clock signal generating means for providing clock signals of a predetermined frequency for transmission over said transmission line to the code generating means of all of said alarm detecting means and to the reference code generating means of all of said tone detecting means to synchronize the operation of said code generating means with the operation of said reference code generating means.

7. An alarm transmission system as set forth in claim 5 wherein said line monitoring means includes separation filter means connected between said transmission line and said tone detecting means including means for passing the tone sequence provided by each tone generator means to an associated tone detecting means.

8. An alarm transmission system as set forth in claim 6 wherein said comparator means includes first and second bit storage means, said clock signals being extended to said first and second bit storage means to gate the bits of the received line supervisory code into said first bit storage means a bit at a time and to gate corresponding bits of the reference code into said second bit storage means a bit at a time, and gating means for comparing the bits stored in the first and second bit storage means and providing an output signal whenever the stored bits are different.

9. An alarm transmission system as set forth in claim 8 wherein said tone detecting means includes alarm output means enabled by each output signal provided by the gating means to provide an alarm indication at the central monitor.

10. An alarm transmission system as set forth in claim 5 wherein said control means includes intrusion detector means operable to provide an alarm output signal in response to an unauthorized entry of the corresponding protected area, and gating means normally enabled to pass the bits of the supervisory code to the tone generating means to effect the generation of the supervisory code tone sequence, said gating means being responsive to each alarm output signal to inhibit the passage of the bits of the supervisory code to the tone generating means whereby the tone generating means provides a continuous tone of one of the predetermined frequencies.

1 1. An alarm transmission system as set forth in claim 5 wherein said alarm detecting means includes status means for controlling said tone generating means to be operable in a first mode to generate a tone sequence having signals of a first pair of frequencies and to be operable in a second mode to generate a tone sequence having signals of a second pair of frequencies, each of said tone detecting means including status detecting means responsive to a sequence of signals of said first pair of frequencies to provide an indication that intrusions of the protected area are authorized and responsive to a sequence of signals of said second pair of frequencies to provide a further indication that intrusions of said protected area are unauthorized.

12. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, an intrusion detector at said protected area operable to provide an alarm output in response to an intrusion of the protected area, alarm detecting means including code continuously generating means for generating a line supervisory code comprised of a known sequence of coded bits, tone generating means normally controlled by the bits of said line supervisory code to generate tones of first and second frequencies in a sequence coded to represent the bits of said supervisory code for transmission over said transmission line to said central monitor, and means responsive to each alarm output for controlling said tone generating means to provide a continuous tone output of one of said frequencies, and line monitoring means at the central monitor including means for detecting the tone sequence provided by said alarm detecting means and means responsive to a continuous tone output provided in response to an intrusion of the protected area to provide an alann indication at said central monitor.

13. In a security system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, intrusion detector means at said protected area operable to provide an alarm output in response to an intrusion of said protected area, alarm detecting means including means for continuously generating a line supervisory code comprised of a known sequence of coded bits, tone generating means responsive to the bits of the supervisory code to provide tone signals of predetermined frequencies in a sequence coded to represent the bits of the supervisory code for transmission over said transmission line to said central monitor, status means for selectively controlling said tone generating means to generate a first tone sequence having signals of a first pair of frequencies to indicate that instrusions of the protected area are authorized and to generate a second tone sequence having signals of a second pair of frequencies to indicate that intrusions of the protected area are unauthorized, and control means responsive to each alarm output provided by said intrusion detector means for controlling said tone generating means to modify the tone sequence being provided by said tone generating means to indicate the detection of an intrusion of the protected area, and line monitoring means at said central monitor including tone detecting means for receiving the tone sequence provided by said alarm detecting means, first means enabled by said tone detecting means whenever a modified tone sequence is provided by said tone generating means to provide an output indicating an intrusion of the protected area and second means enabled by said tone detecting means for indicating whether the intrusion is authorized or unauthorized.

14. An alarm transmission system as set forth in claim 13 wherein said tone generating means includes trilevel oscillator means operable in a first mode to provide signals of first and second frequencies and operable in a second mode to provide signals of said second frequency and a third frequency.

15. An alarm transmission system as set forth in claim 14 wherein said tone generating means further includes bias supply means responsive to the bits of said supervisory code to provide control signals for said oscillator means, said status means including switch means for selectively controlling said bias supply means to provide control signals at first and second levels for said oscillator means to render said oscillator means operable in said first mode and control signals at third and fourth levels for said oscillator means to render said oscillator means operable in said second mode.

16. An alarm transmission system as set forth in claim 15 wherein said tone detecting means includes means responsive to the supervisory code sequence of said first and second frequencies to provide a first sequence of output signals having first and second voltage levels and responsive to the supervisory code sequence of said second and third frequencies to provide a second sequence of output signals having third and fourth voltage levels.

17. An alarm transmission system as set forth in claim 15 wherein said second means includes bistable switching means responsive to an output signal at said first voltage level to provide an output indicating that intrusions of the protected area are authorized and responsive to an output signal at said fourth voltage level to provide an output indicating that intrusions of the protected area are unauthorized.

18. An alarm transmission system as set forth in claim 14 wherein said status switch means further includes tamper switch means for normally connecting the output of said oscillator means to said transmission line and operable responsive to tampering with said status switch means to disconnect the output of said oscillator means from said transmission line thereby interrupting the transmission of said supervisory code tone sequence to said central monitor.

19. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, intrusion detector means at said protected area operable to provide an alarm output in response to an intrusion of said protected area, alarm detecting means associated with said intrusion detector means including output means operable in a first mode to continuously provide a supervisory code comprised of signals of a first pair of frequencies in a predetermined sequence for transmission over said transmission line to said central monitor and operable in a second mode to provide a supervisory code comprised of signals of a second pair of frequencies in a predetermined sequence for transmission over said transmission line to said central monitor, status means for selecting the mode of operation of said output means and means responsive to each alarm output provided by said intrusion detector means to modify the sequence of frequency signals being provided by said output means to indicate the detection of an intrusion of the protected area, and line monitoring means at said central monitor including signal detecting means for receiving the sequence of coded frequency signals provided by said output means, status detecting means controlled by said signal detecting means to provide an indication that intrusions of the protected area are authorized whenever a sequence of signals of said first pair of frequencies is being received and to provide a further indication that intrusions of the protected area are unauthorized whenever a sequence of signals of said second pair of frequencies is being received, and alarm indicating means enabled whenever a modified sequence of frequency signals is received to provide an output indicating the detection of an intrusion of the protected area.

20. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, alarm source means including intrusion detector means at said protected area operable to provide an alarm output in response to the detection of movement of an intruder within the protected area, alarm source monitoring means including alarm detecting means having output means for continuously generating a line supervisory code comprised of a selected sequence of tone signals of predetermined frequencies and control means responsive to each alarm output for controlling said output means to supervisory code tone sequence to indicate the detection of movement of an intruder within the protected area, and line monitoring means at said central monitor including means responsive to a modified supervisory code tone sequence to provide an alarm indication at said central monitor, and test signal generating means operable to provide a test signal for transmission over said transmission line to said alarm source monitoring means, said alarm source monitoring means further including test signal detecting means for detecting said test signal and motion simulator means enabled by said test signal detecting means whenever a test signal is received to enable said intrusion detecting means to provide an alarm output for enabling a modified supervisory code tone sequence to be provided for transmission to said central monitor.

21. An alarm transmission system as set forth in claim wherein said intrusion detector means includes microwave motion detection means having means operable to produce Doppler alarm signals of frequencies within a predetermined range whenever a human intecting means for each of said protected areas, each of said alarm detecting means including code generating means for continuously generating a line supervisory code comprised of a known sequence of .coded bits, tone generating means normally controlled by the bits of said supervisory code to generate tones of at least first and second frequencies in a' sequence coded to represent the bits of the supervisory code for transmission over said transmission line to the central monitor, means for controlling Said tone generating means to provide a continuous tone output of one of said frequencies to indicate an alarm for the corresponding protected area, the tone generating means of different alarm detecting means providing continuously tone sequences having signals of different frequencies to enable simultaneous transmission of the tone sequences provided by all of said tone generating means to said central monitor over the transmission line, and coupling means for coupling the tone sequences provided by all of said tone generating means to said transmission line, and line monitoring means at said central monitor including a separate tone detecting means for each of said alarm detecting means and filter means for extending the tone sequences provided by the tone generating means of each alarm detecting means to a corresponding tone detecting means, each of said tone detecting means including reference code generating means for generating a reference code comprised of a sequence of bits in which each bit is normally identical with corresponding bits of the supervisory code sequence provided by the code generating means of the corresponding alarm detecting means, code comparator means for comparing the bits of the reference code with corresponding bits of the supervisory code, and means controlled by the code comparator means for providing an alarm indication whenever any of the corresponding bits of the two sequences are different.

23. An alarm transmission system as set forth in claim 22 wherein said coupling means comprises summing amplifier means having a plurality of inputs, each indi- 'vidually connected to the output of one of said tone generating means, and an output connected to said transmission line.

24. An alarm transmission system as set forth in claim 22 wherein said filter means comprises a plurality of bandpass filters each individually connected between said transmission line and one of said tone detecting means, different ones of said bandpass filters being tuned to pass signals of the first and second frequencies provided by the associated alarm detecting means.

25. in an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, alarm source means including an individual alarm sensor for each of said protected areas, alarm source monitoring means including at least one code generating means for continuously generating a line supervisory code comprised of a known sequence of coded bits, a plurality of tone generators and a plurality of gating means, each of said gating means being individually associated with one of said tone generators and normally operable to extend the bits of the supervisory code to an associated tone generator, each of said tone generators being responsive to the bits of the supervisory code extended thereto to provide tone signals of at least first and second frequencies in a sequence coded to represent the bits of the supervisory code, different ones of said tone generators being operable to provide tone signals of different frequencies to enable simultaneous transmission over said transmission line of the tone sequences provided by all of said tone generating means, each of said gating means being individually connected to one of said alarm sensors and controlled by an associated alarm sensor to inhibit the passage of the bits of the supervisory code to the associated tone generator to thereby modify the tone sequence to indicate an alarm condition for the corresponding protected area, and line monitoring means at said central monitor including a separate tone detector corresponding to each of said tone generators for receiving the tone sequence provided by an associated tone generator, at least one reference code generating means for generating a reference code comprised of a selected sequence of coded bits, each bit of which is normally identical with corresponding bits of the supervisory code sequence at any given time, and a plurality of code comparator circuits each operable to compare the supervisory code received by an associated one of the tone detectors with the reference code and provide an indication whenever .the codes are different.

26. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, supervisory code generating means at said protected area for continuously generating a line supervisory code comprising a selected sequence of code bits for transmission over the transmission line from said protected area to said central monitor, normally closed contact means interposed between said supervisory code generating means and said transmission line for interrupting the transmission of said supervisory code sequence whenever said contact means are operated to indicate an alarm, reference code generating means at said central monitor for generating a reference code comprised of a selected sequence of code bits that is normally identical to said supervisory code sequence, means for comparing said supervisory code with said reference code, and means for providing an alarm indication whenever the codes are different or the transmission of said supervisory code sequence is interrupted.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,792,470 Dated February 12, 1974 John C. Donovan, Carl F. Klein, Lawrence B. Korta n iyer Patent No.

Inventofls) It: iscertified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 24, line 51, after "to" and before "311-" insert modify the Column 25, line 30, "continuously" should be continuous Signed and sealed this 1st day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN- Commissioner of Patents Attesting Officer 

1. In an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, alarm detecting means for each of said protected areas, each of said alarm detecting means including output means for continuously generating a line supervisory code comprised of a selected sequence of tone signals of predetermined frequencies, different output means being operable to provide supervisory code sequences having signals of different frequencies to enable simultaneous transmission over said transmission line of the supervisory code tone sequences provided by all of the output means, and separate control means for each output means for controlling an associated output means to modify the supervisory code tone sequence to indicate an alarm for a corresponding protected area, and line monitoring means at said central monitor, including an individual tone detecting means for each of said alarm detecting means including receiver means for receiving the supervisory code tone sequences provided by an associated alarm detecting means and alarm output means responsive to a modified supervisory code tone sequence provided by the associated alarm detecting means to provide an alarm indication at said central monitor.
 2. An alarm transmission system as set forth in claim 1 wherein each said output means includes signal generating means for generating tone signals of first and second frequencies in said selected sequence, said signal generating means being controlled by said control means to provide a continuous tone output of one of said frequencies to indicate an alarm.
 3. An alarm transmission system as set forth in claim 1 wherein said output means includes code generating means for generating a supervisory code comprised of a selected sequence of coded logic level bits, and tone generating means responsive to the bits of the supervisory code sequence to normally provide tone signals of predetermined frequencies in a sequence coded to represent the bits of the monitor code, and wherein each of said receiver means at said central monitor includes tone converter means for converting the received tone sequence into a sequence of logic level bits, reference code generating means for generating a reference code comprised of a sequence of logic level bits in which each bit is normally identical with a corresponding bit of the supervisory code at any given time, and each of said alarm output means includes comparator means for comparing each bit of the supervisory code with the coresponding bit of the reference code and means for providing an indication whenever the corresponding bits are different.
 4. An alarm transmission system as set forth in claim 3 wherein each of said control means includes gating means normally enabled to pass tHe bits of the supervisory code unaltered to the corresponding output means to effect the generation of the supervisory code tone sequence, said control means further including alarm sensor means operable to provide an alarm output signal indicating an alarm condition in the corresponding protected area, said gating means being responsive to an alarm output signal provided by an associated alarm sensor means to invert each bit of the supervisory code as the bits are passed to the tone generating means to thereby effect the generation of said modified supervisory code tone sequence.
 5. In an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, alarm detecting means for each of said protected areas, each of said alarm detecting means including code generating means for continuously generating a line supervisory code comprised of a selected sequence of logic level coded bits, tone generating means responsive to the bits of the supervisory code sequence to normally provide tone signals of at least first and second frequencies in a sequence coded to represent the bits of the supervisory code, and separate control means for each alarm generating means for controlling an associated tone generating means to modify the tone sequence to indicate an alarm for the corresponding protected area, different tone generating means being operable to provide tone signals of different frequencies to enable simultaneous transmission over said transmission line of the tone sequences provided by all of the tone generating means, and line monitoring means at said central monitor, including an individual tone detecting means for each of said alarm detecting means for receiving the tone sequences provided by an associated tone generating means, each of said tone detecting means including tone converter means for converting the received tone sequence into a sequence of logic level bits, reference code generating means for generating a reference code comprised of a sequence of logic level bits in which each bit is normally identical with a corresponding bit of the supervisory code at any given time, and comparator means for comparing each bit of the supervisory code with the corresponding bit of the reference code and providing an indication whenever any of the corresponding bits of the two sequences are different.
 6. An alarm transmission system as set forth in claim 5 wherein said line monitoring means further includes clock signal generating means for providing clock signals of a predetermined frequency for transmission over said transmission line to the code generating means of all of said alarm detecting means and to the reference code generating means of all of said tone detecting means to synchronize the operation of said code generating means with the operation of said reference code generating means.
 7. An alarm transmission system as set forth in claim 5 wherein said line monitoring means includes separation filter means connected between said transmission line and said tone detecting means including means for passing the tone sequence provided by each tone generator means to an associated tone detecting means.
 8. An alarm transmission system as set forth in claim 6 wherein said comparator means includes first and second bit storage means, said clock signals being extended to said first and second bit storage means to gate the bits of the received line supervisory code into said first bit storage means a bit at a time and to gate corresponding bits of the reference code into said second bit storage means a bit at a time, and gating means for comparing the bits stored in the first and second bit storage means and providing an output signal whenever the stored bits are different.
 9. An alarm transmission system as set forth in claim 8 wherein said tone detecting means includes alarm output means enabled by each output signal provided by the gating means to pRovide an alarm indication at the central monitor.
 10. An alarm transmission system as set forth in claim 5 wherein said control means includes intrusion detector means operable to provide an alarm output signal in response to an unauthorized entry of the corresponding protected area, and gating means normally enabled to pass the bits of the supervisory code to the tone generating means to effect the generation of the supervisory code tone sequence, said gating means being responsive to each alarm output signal to inhibit the passage of the bits of the supervisory code to the tone generating means whereby the tone generating means provides a continuous tone of one of the predetermined frequencies.
 11. An alarm transmission system as set forth in claim 5 wherein said alarm detecting means includes status means for controlling said tone generating means to be operable in a first mode to generate a tone sequence having signals of a first pair of frequencies and to be operable in a second mode to generate a tone sequence having signals of a second pair of frequencies, each of said tone detecting means including status detecting means responsive to a sequence of signals of said first pair of frequencies to provide an indication that intrusions of the protected area are authorized and responsive to a sequence of signals of said second pair of frequencies to provide a further indication that intrusions of said protected area are unauthorized.
 12. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, an intrusion detector at said protected area operable to provide an alarm output in response to an intrusion of the protected area, alarm detecting means including code continuously generating means for generating a line supervisory code comprised of a known sequence of coded bits, tone generating means normally controlled by the bits of said line supervisory code to generate tones of first and second frequencies in a sequence coded to represent the bits of said supervisory code for transmission over said transmission line to said central monitor, and means responsive to each alarm output for controlling said tone generating means to provide a continuous tone output of one of said frequencies, and line monitoring means at the central monitor including means for detecting the tone sequence provided by said alarm detecting means and means responsive to a continuous tone output provided in response to an intrusion of the protected area to provide an alarm indication at said central monitor.
 13. In a security system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, intrusion detector means at said protected area operable to provide an alarm output in response to an intrusion of said protected area, alarm detecting means including means for continuously generating a line supervisory code comprised of a known sequence of coded bits, tone generating means responsive to the bits of the supervisory code to provide tone signals of predetermined frequencies in a sequence coded to represent the bits of the supervisory code for transmission over said transmission line to said central monitor, status means for selectively controlling said tone generating means to generate a first tone sequence having signals of a first pair of frequencies to indicate that instrusions of the protected area are authorized and to generate a second tone sequence having signals of a second pair of frequencies to indicate that intrusions of the protected area are unauthorized, and control means responsive to each alarm output provided by said intrusion detector means for controlling said tone generating means to modify the tone sequence being provided by said tone generating means to indicate the detection of an intrusion of the protected area, and line monitoring means at said central monitor including tone detecting means for receiving the tone sequence provided by said alarm detecting means, first means enabled by said tone detecting means whenever a modified tone sequence is provided by said tone generating means to provide an output indicating an intrusion of the protected area and second means enabled by said tone detecting means for indicating whether the intrusion is authorized or unauthorized.
 14. An alarm transmission system as set forth in claim 13 wherein said tone generating means includes trilevel oscillator means operable in a first mode to provide signals of first and second frequencies and operable in a second mode to provide signals of said second frequency and a third frequency.
 15. An alarm transmission system as set forth in claim 14 wherein said tone generating means further includes bias supply means responsive to the bits of said supervisory code to provide control signals for said oscillator means, said status means including switch means for selectively controlling said bias supply means to provide control signals at first and second levels for said oscillator means to render said oscillator means operable in said first mode and control signals at third and fourth levels for said oscillator means to render said oscillator means operable in said second mode.
 16. An alarm transmission system as set forth in claim 15 wherein said tone detecting means includes means responsive to the supervisory code sequence of said first and second frequencies to provide a first sequence of output signals having first and second voltage levels and responsive to the supervisory code sequence of said second and third frequencies to provide a second sequence of output signals having third and fourth voltage levels.
 17. An alarm transmission system as set forth in claim 15 wherein said second means includes bistable switching means responsive to an output signal at said first voltage level to provide an output indicating that intrusions of the protected area are authorized and responsive to an output signal at said fourth voltage level to provide an output indicating that intrusions of the protected area are unauthorized.
 18. An alarm transmission system as set forth in claim 14 wherein said status switch means further includes tamper switch means for normally connecting the output of said oscillator means to said transmission line and operable responsive to tampering with said status switch means to disconnect the output of said oscillator means from said transmission line thereby interrupting the transmission of said supervisory code tone sequence to said central monitor.
 19. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, intrusion detector means at said protected area operable to provide an alarm output in response to an intrusion of said protected area, alarm detecting means associated with said intrusion detector means including output means operable in a first mode to continuously provide a supervisory code comprised of signals of a first pair of frequencies in a predetermined sequence for transmission over said transmission line to said central monitor and operable in a second mode to provide a supervisory code comprised of signals of a second pair of frequencies in a predetermined sequence for transmission over said transmission line to said central monitor, status means for selecting the mode of operation of said output means and means responsive to each alarm output provided by said intrusion detector means to modify the sequence of frequency signals being provided by said output means to indicate the detection of an intrusion of the protected area, and line monitoring means at said central monitor including signal detecting means for receiving the sequence of coded frequency signals provided by said output means, status detecting means controlled by said signal detecting means tO provide an indication that intrusions of the protected area are authorized whenever a sequence of signals of said first pair of frequencies is being received and to provide a further indication that intrusions of the protected area are unauthorized whenever a sequence of signals of said second pair of frequencies is being received, and alarm indicating means enabled whenever a modified sequence of frequency signals is received to provide an output indicating the detection of an intrusion of the protected area.
 20. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, alarm source means including intrusion detector means at said protected area operable to provide an alarm output in response to the detection of movement of an intruder within the protected area, alarm source monitoring means including alarm detecting means having output means for continuously generating a line supervisory code comprised of a selected sequence of tone signals of predetermined frequencies and control means responsive to each alarm output for controlling said output means to supervisory code tone sequence to indicate the detection of movement of an intruder within the protected area, and line monitoring means at said central monitor including means responsive to a modified supervisory code tone sequence to provide an alarm indication at said central monitor, and test signal generating means operable to provide a test signal for transmission over said transmission line to said alarm source monitoring means, said alarm source monitoring means further including test signal detecting means for detecting said test signal and motion simulator means enabled by said test signal detecting means whenever a test signal is received to enable said intrusion detecting means to provide an alarm output for enabling a modified supervisory code tone sequence to be provided for transmission to said central monitor.
 21. An alarm transmission system as set forth in claim 20 wherein said intrusion detector means includes microwave motion detection means having means operable to produce Doppler alarm signals of frequencies within a predetermined range whenever a human intruder is moving within the protected area and alarm signal detecting means responsive to each of said Doppler alarm signals to provide an alarm output, said motion simulator means being operable when enabled to produce signals of a frequency within said predetermined range to enable said motion detection means to provide an alarm output.
 22. In an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, an individual alarm detecting means for each of said protected areas, each of said alarm detecting means including code generating means for continuously generating a line supervisory code comprised of a known sequence of coded bits, tone generating means normally controlled by the bits of said supervisory code to generate tones of at least first and second frequencies in a sequence coded to represent the bits of the supervisory code for transmission over said transmission line to the central monitor, means for controlling said tone generating means to provide a continuous tone output of one of said frequencies to indicate an alarm for the corresponding protected area, the tone generating means of different alarm detecting means providing continuously tone sequences having signals of different frequencies to enable simultaneous transmission of the tone sequences provided by all of said tone generating means to said central monitor over the transmission line, and coupling means for coupling the tone sequences provided by all of said tone generating means to said transmission line, and line monitoring means at said central monitor including a separate tone detecting means for each of sAid alarm detecting means and filter means for extending the tone sequences provided by the tone generating means of each alarm detecting means to a corresponding tone detecting means, each of said tone detecting means including reference code generating means for generating a reference code comprised of a sequence of bits in which each bit is normally identical with corresponding bits of the supervisory code sequence provided by the code generating means of the corresponding alarm detecting means, code comparator means for comparing the bits of the reference code with corresponding bits of the supervisory code, and means controlled by the code comparator means for providing an alarm indication whenever any of the corresponding bits of the two sequences are different.
 23. An alarm transmission system as set forth in claim 22 wherein said coupling means comprises summing amplifier means having a plurality of inputs, each individually connected to the output of one of said tone generating means, and an output connected to said transmission line.
 24. An alarm transmission system as set forth in claim 22 wherein said filter means comprises a plurality of bandpass filters each individually connected between said transmission line and one of said tone detecting means, different ones of said bandpass filters being tuned to pass signals of the first and second frequencies provided by the associated alarm detecting means.
 25. In an alarm transmission system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitor that is remote from the protected areas, alarm source means including an individual alarm sensor for each of said protected areas, alarm source monitoring means including at least one code generating means for continuously generating a line supervisory code comprised of a known sequence of coded bits, a plurality of tone generators and a plurality of gating means, each of said gating means being individually associated with one of said tone generators and normally operable to extend the bits of the supervisory code to an associated tone generator, each of said tone generators being responsive to the bits of the supervisory code extended thereto to provide tone signals of at least first and second frequencies in a sequence coded to represent the bits of the supervisory code, different ones of said tone generators being operable to provide tone signals of different frequencies to enable simultaneous transmission over said transmission line of the tone sequences provided by all of said tone generating means, each of said gating means being individually connected to one of said alarm sensors and controlled by an associated alarm sensor to inhibit the passage of the bits of the supervisory code to the associated tone generator to thereby modify the tone sequence to indicate an alarm condition for the corresponding protected area, and line monitoring means at said central monitor including a separate tone detector corresponding to each of said tone generators for receiving the tone sequence provided by an associated tone generator, at least one reference code generating means for generating a reference code comprised of a selected sequence of coded bits, each bit of which is normally identical with corresponding bits of the supervisory code sequence at any given time, and a plurality of code comparator circuits each operable to compare the supervisory code received by an associated one of the tone detectors with the reference code and provide an indication whenever the codes are different.
 26. In an alarm transmission system including a transmission line for carrying alarm information from at least one protected area to a central monitor that is remote from the protected area, supervisory code generating means at said protected area for continuously generating a line supervisory code comprising a selected sequence of code bits for transmission over the transmission line from said protected area to said central monitor, normally closed contact means interposed between said supervisory code generating means and said transmission line for interrupting the transmission of said supervisory code sequence whenever said contact means are operated to indicate an alarm, reference code generating means at said central monitor for generating a reference code comprised of a selected sequence of code bits that is normally identical to said supervisory code sequence, means for comparing said supervisory code with said reference code, and means for providing an alarm indication whenever the codes are different or the transmission of said supervisory code sequence is interrupted. 